WASHINGTON — New York police will mount a major counterterrorism operation during Friday’s evening rush hour as U.S. officials rush to check credible, but unconfirmed intelligence about a 9/11 anniversary plot, a senior law enforcement official told Reuters.

The operation will involve a “big show of force” which will include officers armed with heavy weapons patrolling subways and stepped up security and bag checks at major subway and commuter train stations, including Grand Central and Pennsylvania station terminals, the official said. Law enforcement agencies in neighboring states were also involved.

While the operation had been planned some time ago, the forces and tactics deployed will be stepped up after local and federal authorities received what they described as “specific and credible” but unconfirmed intelligence about a possible threat to attack the United States by Islamic militants timed to coincide with the 10th anniversary of the September 11, 2001, al-Qaeda attacks on New York and Washington.

Intelligence about the anniversary threat only surfaced during the course of this week and included possible threats of attacks targeting subways or commuter trains or possible car bomb attacks in New York or Washington, U.S. officials said.

Vice President Joseph Biden said on ABC’s “Good Morning America” program on Friday morning: “We don’t have the smoking gun but we do have talk about using a car bomb.”

One of the officials said the threat is believed to come from militants following an al-Qaeda ideology. But it is not certain whether there is a direct connection to what remains of al-Qaeda’s battered core leadership.

Officials have said that U.S. law enforcement agencies are on the lookout for three possible suspects, though it is unclear how much, if any, precise information regarding the suspects’ identities is available to U.S. authorities.

“We have to be concerned. Terrorism is theater and this is a stage, right now probably the world’s biggest stage,” said New York City Police Commissioner Raymond Kelly.

“We have the opening of the 9/11 memorial, the president and two former presidents here, obviously a lot of high profile public officials will be here, so we have to be concerned,” Kelly told CNN.

On Thursday, President Barack Obama ordered a redoubling of U.S. counterterrorism efforts in the face of a “credible but unconfirmed” threat before the September 11 attacks anniversary.

U.S. officials, speaking to Reuters on condition of anonymity, said the threat involved Washington and New York City, which were targeted in al-Qaeda attacks a decade ago this Sunday that killed nearly 3,000 people.

A counterterrorism official said intelligence pointed to possible car bomb attacks and added the threat information came from Pakistan’s tribal areas. A manhunt was under way for two or three suspects, who one person familiar with the matter said were suspected of having links to al-Qaeda.

But U.S. officials used strong caveats when discussing the threat information privately, with a national security official cautioning that experts thought the threat would ultimately not check out.

New York City Mayor Michael Bloomberg stressed at a news conference on Friday that the threat had not been corroborated, even as he announced heightened security measures “some of which you may notice, some of which you may not notice.”

Bloomberg asked citizens to go about their daily routines as usual but report any suspicious or dangerous activity. “Over the next three days we should all keep our eyes wide open.”

The White House said Obama was briefed on specific threat information on Thursday morning, and said the government had already heightened security before the anniversary.

Documents discovered in Osama bin Laden’s compound in Abbottabad, Pakistan, after he was killed in a raid in May by Navy SEALs, highlighted his persistent interest in attacking the United States around the anniversary of the 2001 attacks. But it is unclear if the plans ever evolved beyond aspiration.

“As we know from the intelligence gathered following the Osama bin Laden raid, al-Qaeda has showed an interest in important dates and anniversaries, such as 9/11,” said Jan Fedarcyk with the FBI’s New York field office.

Kelly announced measures including more bag inspections on the subway, more bomb-sniffing dogs on patrol and increased deployment of radiation monitoring equipment.

“There will be increased focus on tunnels and bridges and infrastructure in general, as well as landmark locations, houses of worship and government buildings,” he said.

A law-enforcement source played down an ABC News report about missing rental trucks — saying the vehicles had been recovered and there was no connection to terrorism.

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